1. Field of the Invention
The present invention relates to a microoptical system which is one of the arts for interconnection between electronics chips used for free-space optical interconnection and its setting method.
2. Description of the Related Art
The free-space optical interconnection is noticed because of the large bandwidth characteristic of light and the profitability of parallel processing. Moreover, it is known that a packaged optical system has a high reliability and it can be made compact in the art of interconnection between chip modules used for the free-space interconnection. One of the packaged optical systems is disclosed in Document I: "Optical Engineering" Vol. 33, pp. 1550-1560 (1994)".
Generally, in the case of the free-space optical interconnection, signals are propagated and combined in the form of light by using a lens and a mirror and making the most use of the degree of freedom of a three-dimensional space without using any optical fiber or optical waveguide. The optical interconnection system includes the following three types as shown in FIGS. 11(A), 11(B), and 11(C):
&lt;1&gt; a macrooptical system for optically imaging an element array or one chip module S comprising point-light-source elements s on an element array or the other chip module D comprising photodetector elements d in batch by common macro lenses M1 and M2 {FIG. 11(A)}, PA1 &lt;2&gt; a hybrid optical system in which each point-light-source element s is collimated by a lens m1 and each photodetector element d is collimated by a lens m2, and they are connected each other by being optically imaged through macro lenses M1 and M2 {FIG. 11(B)}, and PA1 &lt;3&gt; a microoptical system in which each point-light-source element s is collimated by a lens m1 and each photodetector element d is collimated by a lens m2, and these collimated beams are directly connected each other at an equal beam radius without using any microlens or macro lenses. PA1 .omega..sub.1 --beam waist of Gaussian beams of point-light-source element; PA1 .omega..sub.2 --effective Gaussian-beam radius of first microlens; PA1 .omega..sub.3 --beam waist of Gaussian beams of intermediate image of point-light-source element after first microlens; PA1 .omega..sub.4 --effective Gaussian-beam radius of second microlens; PA1 .omega..sub.5 --beam waist of Gaussian beams at light receiving plane of photodetector element; PA1 L.sub.1 --distance from point-light-source element to first microlens; PA1 L.sub.2 --distance from first microlens to intermediate image; PA1 L.sub.3 --distance from intermediate image to second microlens; PA1 L.sub.4 --distance from second microlens to light receiving plane of photodetector; PA1 f.sub.1 --focal length of first microlens; PA1 f.sub.2 --focal length of second microlens; PA1 .lambda.--wavelength of Gaussian beams; and PA1 .pi.--ratio of circumference of circle to its diameter, and moreover characterized in that:
Among these systems, the microoptical system &lt;3&gt; is more advantageous than two other systems in that any interconnection pattern can be realized.
In the case of the free-space optical interconnection of the microoptical system, it is one of the requisites for design to take a long interconnection length because it is more preferable from the view point of designing the architecture of the system that electronics chips farther separate from each other can be interconnected. In the case of a collimated optical system, however, it is impossible to take a long-enough interconnection length because of the diffraction effect. Therefore, a method for increasing the above optical interconnection length by using an optical system in which two microlenses are arranged so that a beam waist (imaged point) is set between the both microlenses is proposed in the above Document I.
This method for using a Gaussian optical system uses two imaging lenses to perform imaging from a point light source to an intermediate image by the first imaging lens and performs imaging from the intermediate image onto the light receiving plane of a photodetector by the second imaging lens. Moreover, in the case of this method, the value of the beam waist of the intermediate image for maximizing the distance (that is, interconnection length) between the two imaging lenses is obtained as a function of effective beam radiuses of two imaging lenses.
In the case of this microoptical system, however, the request for the arrangement accuracy of a point light source and a photodetector is severe and therefore, a positioning accuracy of approx. 1 to 2 .mu.m has been requested so far.
In general, however, the free-space optical interconnection between electronics chips is complex and any electronics chip is interconnected with other electronics chips. Thus, the conventional method for successively arranging electronics chips while monitoring the incident power of light or amount of light of a photodetector is not proper because it is considered that positioning errors are accumulated. Therefore, it is considered that it is necessary to design an optical-interconnection optical system for combining electronics chips by giving an allowance capable of absorbing point-light-source positional-deviation errors caused by flip chip bonding to the system and individually fixing the chips at an independent positioning accuracy by means of the flip chip boding. In this connection, there is a report that an experimental error of flip chip bonding is approx. 2 to 5 .mu.m (Document II: "IEEE Photonics Technology Letters, Vol.4 pp. 1369-1372, 1992"). Therefore, to mass-produce microoptical systems, the advent of a microoptical system for free-space optical interconnection capable of relaxing the positioning accuracy of each element constituting an electronics chip up to a level adaptable to mass production and its setting method has been desired.